1. Field of the Invention
The present invention relates to power converters, and more particularly, relates to the synchronous rectifier of power converters.
2. Description of the Related Art
FIG. 1 shows a circuit schematic of a soft switching power converter. It includes a transformer 10 to provide isolation from line input VIN to the output VO of the power converter for safety. Switches 20 and 30 develop a half bridge circuit to switch the transformer 10. A switching circuit (SW CIRCUIT) 90 generates switching signals SH and SL for switching the switches 20 and 30 respectively. A leakage inductance of a primary winding NP of the transformer 10 and capacitors 41 and 42 form a resonant tank. The leakage inductance L and the equivalent capacitance C of capacitors 41 and 42 determine the resonance frequency f0 of the resonant tank.
                              f          0                =                  1                      2            ⁢            π            ⁢                                          L                ×                C                                                                        (        1        )            
The transformer 10 transfers the energy from the primary winding NP to the secondary windings NS1 and NS2 of the transformer 10. Rectifiers 61 and 62 and a capacitor 65 perform the rectification and filtering for generating the output VO. A voltage regulation device (such as a zener diode) 70, a resistor 71 and an optical coupler 80 form a regulation circuit coupled to the output VO. The optical coupler 80 is further coupled to the switching circuit 90 for developing the feedback loop of the converter to regulate the output VO. The switching circuit 90 generates the switching signals SH and SL in response to the feedback loop signal.
Although the soft switching power converter can achieve high efficiency and low EMI (electric-magnetic interference) performance, the forward voltage of rectifiers 61 and 62 still causes significant power losses. The object of present invention is to provide the synchronous rectifying circuit for switching power converter to achieve higher efficiency.